VT-d: improve fault info logging

VT-d: improve fault info logging

I got repeatedly annoyed by there not getting anything logged by
default on VT-d faults (and hence having to tell people to add extra
command line options), and hence I think it is time to redo this code:
Log basic fault information at guest-warning level (rate limited by
default), and show the page walk in verbose rather than only in debug
mode. Break up multi-line message so that each gets a proper log level
attached, at once splitting out the common part. Also don't log
"unknown" faults as interrupt-remapping ones.

As a minor cleanup fix the type of the involved "fault_type" variables.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Yang Zhang <yang.z.zhang@intel.com>